1. Field of the Invention
The present invention relates to an optical pickup apparatus for preventing astigmatic aberration.
2. Description of the Related Art
Optical pickup apparatuses for retrieving/recording data from/on optical disks such as CDs or DVDs employ beam splitters to separate the laser beam emitted by a laser diode from the beam reflected by the optical disk or to separate the optical path for CDs from the optical path for DVDs. Most commonly used beam splitters are prism-type beam splitters and plate-type beam splitters.
FIGS. 1A and 1B depict a prism-type beam splitter comprising two identically-shaped prisms attached to each other. The attached surfaces of the prisms are arranged to be inclined at a predetermined angle, for example 45 degrees, with respect to the optical axis and selectively transmit or reflect impinged beams depending upon the polarization thereof. Two differently-polarized beams B1 and B2 incident upon the surface from different angles are combined into a beam B3 in FIG. 1A, whereas an incident beam B1 is divided into two differently-polarized beams B2 and B3 in FIG. 1B.
FIGS. 2A and 2B depict a plate-type beam splitter placed to be inclined at a predetermined angle with respect to the optical axis. Like the prism-type beam splitter, the surfaces of the plate-type beam splitter selectively transmit or reflect impinged beams depending upon the polarization thereof. Therefore, the plate-type beam splitter functions in much the same was as the prism-type beam splitter does, as shown in FIGS. 2A and 2B. The plate-type beam splitter, however, causes astigmatic aberration since the plate-type beam splitter may be misaligned with the incident light beam as shown in FIG. 3.
To retrieve/record data from/on an optical disk, the laser beam emitted by a laser diode should be focused to a single point on the optical disk. The plate-type beam splitters, therefore, are not appropriate for this purpose and accordingly most optical pickups employ prism-type beam splitters positioned in the light-incident path from the laser diode to the optical disk. The plate-type beam splitters are sometimes used to deliberately induce astigmatic aberration in the optical path from the optical disk to optical detectors.
FIGS. 4A and 4B show the optical characteristics of an optical pickup employing the plate-type beam splitter in the light-incident path from the laser diode to the optical disk. FIG. 4A depicts the wave front error (WFE) characteristic versus optical axis tilt and FIG. 4B depicts the interferogram. For normal operation of the optical pickup, the WFE should remain within 0.07xcex as long as the tilt angle remains within xc2x12xc2x0. It is easily seen in FIGS. 4A and 4B that the WFE exceeds the required limit and the interferogram is remarkably distorted, which demonstrates the reason why plate-type beam splitters are not acceptable in the light-incident path of the optical pickup.
Meanwhile, FIGS. 5A and 5B show the optical characteristics of an optical pickup employing the prism-type beam splitter in the light-incident path. Unlike the previous example, it is seen that the WFE curves for tangential and radial are almost identical to each other, the WFE satisfies the required condition, and the interferogram shows little distortion.
The prism-type beam splitters, however, increase manufacturing cost since they are much more expensive than the plate-type beam splitters. Moreover, prism-type beam splitters are constructed by attaching two prisms together, however, this joining work of two prisms is a difficult manufacturing process, and having to attach the two sides sometimes degrade the performance of the prism-type beam splitters.
The invention provides an optical pick apparatus which does not cause astigmatic aberration by employing wedge-shaped beam splitters in the light incident path.
An optical pickup apparatus in accordance with the present invention comprises one or more light sources and beam splitting means which is placed in the light-incident path beginning from the light sources and selectively reflects or transmits the light beams emitted by the light sources, the light-incident side of the beam splitting means not being parallel with the light-exiting side of the beam splitting means. The beam splitting means is a wedge-shaped plate the light-incident side of which is more inclined than the light-exiting side with respect to the optical axis.
Another optical pickup apparatus in accordance with the present invention comprises one or more light sources and wedge-shaped beam splitting means for correcting astigmatic aberration which may arise in the optical path between the light sources and an optical disk. The wedge-shaped beam splitting means is placed between the light sources and the optical disk, projects the incident light beams from the light sources onto the optical disk, and projects the light beams reflected by the optical disk onto an optical detection means.
Another optical pickup apparatus in accordance with the present invention comprises a plurality of light sources and wedge-shaped beam splitting means for refracting incident light beams emitted by the light sources toward different directions.
Another optical pickup apparatus in accordance with the present invention comprises a plurality of light sources placed at different places, wedge-shaped beam splitting means which is placed at the intersection of the light beams emitted by the light sources and refracts incident light beams emitted by the light sources toward different directions, and beam focusing means for focusing the light beam passing through the wedge-shaped beam splitting means on an optical disk.
Another optical pickup apparatus in accordance with the present invention comprises light emitting means and light receiving means placed at different places, first wedge-shaped beam splitting means for refracting incident light beams emitted by the light emitting means and light beams reflected by on optical disk toward different directions, another light source placed at a different place from the light emitting means, second wedge-shaped beam splitting means which is placed at the intersection of the light beams emitted by the light source and the light beams passing through the first wedge-shaped beam splitting means and refracts the light beams emitted by the light source and the light beams passing through the first wedge-shaped beam splitting means toward different directions, and beam focusing means for focusing the light beam passing through the second beam splitting means on the optical disk.